As a conventional exhaust gas analysis apparatus of this type, JP Patent Publication (Kokai) No. 2004-117259 discloses an in-vehicle HC measurement apparatus. The in-vehicle HC measurement apparatus includes the following in-vehicle components: an NDIR (non-dispersive infrared spectroscopy) gas analyzer for continuously measuring the HC (hydrocarbon) concentration in the exhaust gas flowing through the exhaust duct connected to the engine, an exhaust gas flowmeter for continuously measuring the flow of the exhaust gas flowing through the exhaust duct, and an arithmetic processing circuit for computing the outputs of the NDIR gas analyzer and the exhaust gas flowmeter to continuously calculate the amount of THC (total hydrocarbon) in the exhaust gas.
When infrared laser beam-based measurement (infrared absorption method) is applied to the exhaust gas tube, the following problem arises. That is, in this measurement method, the shape of the laser beam absorption spectrum is used to compute and determine the exhaust gas concentration in the tube. On the other hand, the pressure and the concentration in the exhaust gas tube will abruptly change depending on how the engine is operated. When the pressure varies, a phenomenon called broadening changes the shape of the laser beam absorption spectrum, so that the determined concentration value becomes inaccurate.